Most modern buildings have the capability to provide emergency lighting in the event of an interruption to the main power supply. Emergency lighting not only improves safety, but is required by most building codes. Conventional emergency lighting systems work by identifying a failure in the main power supply (or mains power) and switching in a battery backup to supply power to some of the lamps in the building until such time as main power can be restored.
Although simple in concept, there are many issues that emergency lighting systems must resolve. One such issue relates to the fact that most buildings that have emergency lighting use linear fluorescent lamps to provide illumination. Fluorescent lamps are designed to run on alternating current. The batteries that provide emergency power provide direct current. Accordingly, the circuitry, or “ballast,” that provides current from the battery to the fluorescent lamp must convert the battery's direct current into alternating current capable of illuminating the fluorescent lamp.
Many buildings have two different power supplies. For example, in the United States, many commercial buildings have one power supply that operates at 120 volts, and another that operates at 277 volts. Frequently, both supplies will be present in a single junction box. Alternatively, only one supply or the other will be present in a given junction box. During a typical installation process, it may not be possible or convenient to ascertain which voltage is carried on any given wire. For this reason, lighting manufacturers have created universal voltage fixtures for main lighting applications. These universal voltage fixtures are capable of running from either available power supply.
Conventional universal-voltage-fixture circuitry involves the use of an integrated circuit to switch between 120 volts and 277 volts. Because of the expense associated with this solution, however, the feature is not cost effective for emergency ballasts, and therefore is not used. Instead, emergency ballasts require the installer to know the line voltage that will be provided to the emergency ballast. Because this information may not be available, the result of this drawback of conventional emergency ballasts can be additional time in installation or troubleshooting, or even a failure of an emergency ballast that has inadvertently been incorrectly installed.
Conventional emergency ballasts also have a problem when switching between standard operational mode, which is controlled through the standard fluorescent ballast, and emergency mode, which is controlled by the emergency ballast. Certain conventional fluorescent ballasts include a feature for determining whether a lamp is at the end of its usable life. These conventional ballasts do so by detecting whether the attached lamp is in an “open cathode” condition. Once an open cathode is detected, the conventional fluorescent ballast will shut the fixture down. When a power failure occurs, the emergency ballast will identify the power outage and switch to emergency mode.
Once power is resumed, however, conventional emergency ballasts switch control back over to the fluorescent ballast. If this transition occurs quickly, the fluorescent ballast may not have had time to power up properly, causing the fluorescent ballast to identify an open cathode condition and shut down. Then, because neither the emergency ballast nor the fluorescent ballast are operating, the light fixture will be off, potentially necessitating a call to maintenance to remedy the problem.
Conventional emergency ballasts also suffer from an inability to use certain types of rechargeable batteries. Conventional emergency ballasts must be made as inexpensively as possible, or risk being unmarketable. The need to remain inexpensive limits the types of rechargeable batteries that are available for use in emergency ballasts. Specifically, batteries using certain rechargeable technologies, such as nickel cadmium (NiCd), can be charged simply by applying a charge to the battery. Other rechargeable batteries, however, require a more sophisticated charging process.
By way of example only, nickel metal hydride (NiMH) cannot receive a constant voltage. If a charging voltage is applied to NiMH batteries for an extended period after they are charged, the batteries can fail. Conventional charging circuits for NiMH batteries involve the use of an integrated circuit that can handle the sensitive charging needs of a NiMH batteries. The integrated circuit, however, is expensive, and therefore renders it inappropriate for use in emergency ballasts. This is a problem in conventional emergency ballasts, as NiMH technology allows for the use of much smaller batteries that can provide the same amount of power as compared to NiCd batteries.